1. Field of the Invention
This invention relates to a power transmission device which transmits the driving force of a power generator (engine), such as an internal combustion engine and an electric motor, to right and left drive wheels, and relates to a transportation device employing this power transmission device.
2. Description of the Related Art
In a skateboard, a rider can stand on it and can run in a desired direction by putting his or her feet on the board supported by rollers and keeping his or her balance.
Simple transportation devices are being developed and they are utilized as personal transportation means, sport apparatus and leisure equipment. For example, a skateboard with a power generator such as an engine under development is able to run itself easily even on a flat surface. Such devices are still further being developed for improving the turning performance so as to rendering the device easy to control and providing the devices with enough driving capability to qualify as sport apparatus or leisure equipment. Also, some devices are being developed for improving the driving performance so that the devices may run on a rough wasteland as well as on a flat plane.
A differential gear is a common power transmitting device to transmit a driving force generated by an engine to right and left drive wheels and that can absorb the difference in rotation speed of these drive wheels at a turning due to the difference in the number of revolutions between inside and outside wheels. In a transportation device like the skateboard with an engine as described above, if the rider shifts its weight to turn, one of the drive wheels parts from the ground and runs idle. Therefore, if the differential gear is applied in such transportation device, it is impossible to obtain a stable driving force. Also, on a wasteland, one of the drive wheels likely leaves the ground or slips, thus it is impossible to obtain the desired driving force by the differential gear. It is desirable that stable driving capacity be obtained under a variety of conditions such as running on a rough place to enjoy sports, leisure, and the like. In addition, it is important for the user to be able to control the direction easily.
Although it is possible to attach a differential-limiting device to deal with the above problems, the power transmission device becomes large and expensive, which causes the transportation device to be heavy and expensive.
Therefore, an object of this invention is to provide a simple power transmission device having a function of absorbing the difference in the number of revolutions between the inside and outside wheels, and a function transmitting the driving force to the right and left wheels more continuously so as to transmit the stable driving force even when the transportation device inclines while running in a wasteland or curving. Another object of this invention is to provide a power transmission device appropriate for the aforementioned transportation device, which is lightweight, can be provided at a low cost, and has an excellent power transmission property, and to provide a transportation device which, by employing this power transmission device, has high steering performance and driving performance, and which can run in a rough place or a wasteland.